Nuclear fuel assemblies typically include a fuel bundle comprised of a plurality of fuel rods, upper and lower tie plates coupled to the fuel rods, and a fuel bundle channel for receiving the fuel bundle and upper tie plate. A multiplicity of these assemblies, together with control rods, typically comprise a boiling water reactor core. In certain fuel bundle assemblies, the fuel bundle is inserted downwardly into the channel, with the lower tie plate of the bundle resting on top of the channel assembly's nose piece. The upper tie plate is not directly fastened to the channel. Rather, the upper tie plate is positioned and located within the fuel channel by a plurality of identical upwardly projecting, flat, cantilevered springs attached to the sides of the upper tie plate.
Similar additional upwardly projecting cantilever-type springs are employed to position or locate the upper end of the fuel assembly in the multi-bundle cell. These latter springs bias or push the upper end of the fuel assembly toward the centrally located, normally cruciform-shaped control rod such that the fuel assemblies are symmetrically located about the control rod. The movement toward the control rod is constrained by an interaction between adapter blocks in the upper tie plate handle and stub beams attached to the reactor top guide.
In both cases, the springs have been attached to the upper tie plate by small countersunk head-threaded fasteners screwed into the sides of the upper tie plate. Fasteners of this type, however, have failed in operation, resulting in a number of deleterious effects, including accelerated corrosion in the boiling water reactor environment. These failures are believed to be a result of the high stresses applied to the fasteners under ordinary loading conditions and also to differential growth of dissimilar metals. While one solution to the problem may be to weld the fasteners to the tie plate, welding requires extra manufacturing steps and costs.